In the telephone communications industry, air core cable is being replaced by filled cable in particular applications. Each cable core includes a plurality of conductive elements having insulation thereon to form insulated conductors. In filled cable, the interstices of the core are filled with a water-proofing compound to prevent ingress of water into the core which affects the electrical characteristics of the cable. The resplacement of the air in the interstices with water-proofing compound results in inferior dielectric properties. In order to compensate for this, the amount of insulation on the conductive element of each insulated conductor within the core must be increased. This method of construction results in an increase in the cross-sectional area of the insulated conductors as well as that of the core. Moreover, additional sheathing material such as jacketing compound is required to properly cover the core.
In order to realize the advantages of filled cable, the size of the core must be reduced. This may be accomplished by using dual insulation as a primary insulation for the individual conductors. Generally, in dual insulated conductors, a cellular plastic insulation is extruded over and in engagement with the conductive element. A solid abrasion-resistant plastic material is extruded about the cellular plastic insulation to form a skin layer. The reduction in the diameter of the insulated conductor leads to a core size for filled cable which approximates the size of the core in air core cable.
While it is desirable to use cellular plastic insulation, there are certain problems which must be overcome. The processing of an extrudable plastic material containing an expanding medium to form the cellular plastic insulation is a sensitive manufacturing process. The problem of maintaining a predetermined uniform coaxial capacitance between a point on the periphery of the cellular plastic insulation and the insulated conductor is complicated by random variations in the composite dielectric constant of the insulation. These variations may result from changes in the degree of expansion of the cellular plastic insulation which is affected by changes in temperature, pressures and other factors in the process of applying the insulation to the conductive element.
U.S. Pat. No. 3,914,357, which issued to M. R. Cereijo et al., discloses methods of monitoring the application of cellular plastic insulation to an elongated bare conductive element. The method includes advancing successive sections of the conductive element and extruding at least a layer of the cellular insulation material thereon to form an insulated conductor. The capacitance and the associated diameter of the successive section of the insulated conductor are measured. With respect to the weight of insulation per length of conductor and the percent voids in the cellular insulation material, a continuous indication of the measured capacitance and the associated diameter of the successive sections is generated. The percent expansion or the weight of the insulation material per length of the elongated material may be regulated in response to the generated indication to maintain the diameter and the associated capacitance substantially at preselected values.
In view of the foregoing, there is a need for a system which accurately and precisely controls coaxial capacitance of the insulated conductor by using the measured capacitance to control the temperature of the bare conductive element entering an extruder and the point of cooling or quenching the insulated conductor.